Only some emails arriving into Zoho CRM Email

Only some emails arriving into Zoho CRM Email

Hi, have been looking to switch over to using emails on our Zoho CRM and have had partial success.

Have noticed however that not all the emails arrive into the zoho crm email? It seems to be larger emails and newsletter however this is not a consistent rule. I also get emails in Outlook which is where I'm hoping to migrate from and it seems to get all messages. 

Really can't look to migrate until I can get this is sorted. 

A thought I had whilst typing this, is that maybe it is some of the following;
- POP account settings in Outlook is set to delete messages from server upon arrival. (Have changed this now to see if that makes a difference)
- I look at the emails in both the 'Zoho CRM Email' module and also 'Zoho Mail'. Not sure if one upsets the other, but will try to not do this for a while.

Any thoughts?